The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for transmission ratio of the transmission at low vehicle speed.
A known continuously variable belt-drive transmission for a motor vehicle (Japanese Patent Laid Open No. 55-65755) comprises an endless belt running over a drive pulley and a driven pulley. Each pulley comprises a movable conical disc which is axially moved by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a hydraulic circuit including a pump for supplying oil to servo devices, a line pressure control valve and a transmission ratio control valve. Each valve comprises a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The output of the engine is transmitted to the drive pulley through a clutch. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
In such a system, when the vehicle is decelerated and vehicle speed reduces below a predetermined speed, the clutch is disengaged in order to prevent stalling of the engine. At transmission ratio is increased to increase the speed of a drive pulley of the transmission so that the drive pulley speed equals the speed of the engine transmission ratio and the vehicle speed. Thereafter, when an accelerator pedal of the vehicle is depressed in order to re-accelerate the vehicle, engine speed is increased and hence speed of a drive member of the clutch is increased. When the engine speed exceeds a predetermined speed, the clutch is engaged. However, there is a difference between the high engine speed and the low drive-pulley speed, which causes occurrence of a shock upon the engagement of the clutch.
Japanese Patent Laid Open No. 60-139925 discloses a system to smoothly engage an electromagnetic clutch at the re-acceleration of a vehicle. The system is adapted to engage the clutch when the engine speed coincides with the speed of a driven member of the clutch. However, the engagement of the clutch is performed regardless of load on the engine, that is the depression degree of the accelerator pedal. Accordingly, the shock at the engagement can not be entirely eliminated.
Japanese Patent Laid Open No. 60-98257 discloses a transmission ratio control system at re-acceleration of a vehicle. The system is arranged to increase the transmission ratio of a transmission to increase the speed of a drive pulley. When the engine speed coincides with the speed of the drive pulley, a clutch is engaged. However, there is a time delay until the clutch engages, since the clutch is engaged after the downshifting of the transmission at the re-acceleration of the vehicle.